Feller buncher machines in use today in the timber harvesting business are commonly either one of two types. A first type comprises a four-wheel drive tractor with a bunching shear or bunching saw attachment mounted on its front end. An example of this first type of feller buncher machine is the HYDRO-AX Model 511 EX machine manufactured and sold by the Foresting and Industrial Equipment Division of Blount, Inc. in Owatonna, Minn. A second type comprises a tracked vehicle mounting a rotatable boom which carries a bunching shear, bunching saw or delimbing device or the like at its free end. An example of this second type of feller buncher machine is illustrated in the Crawford U.S. Pat. No. 4,583,908.
In the second type of machine, a boom platform is rotatably mounted on a turntable supported by a chassis. The chassis is supported by a pair of endless tracks. The endless tracks are independently driven by hydrostatic motors. The boom platform mounts a boom having a bunching shear or tree delimbing device or some other tool at its free end. The platform and, thus, the boom are rotatable through an arc of 360.degree. about the axis of an annular ring bearing on the turntable which supports the platform.
In the machine illustrated in the Crawford patent, the boom is pivotally mounted on the rear of the platform, outside of a circle defined by the ring bearing. The actuator cylinder for the boom is pivotally mounted on the platform rearwardly of the platform's axis of rotation, but inside the circle defined by the ring bearing. As a result, the loads concentrated at the boom base pin and the actuator cylinder base pin are not balanced relative to the supporting ring bearing. Pin load reaction on the boom platform is not balanced. The bearing ring is subjected to widely varying stresses at different locations as the boom platform rotates. Unequal stress in the bearing and the platform requires heavier components and results in undue wear and shortened service life.